The invention relates to the manufacture of rotary drill bits for use in drilling or coring holes in subsurface formations.
The invention is applicable to the manufacture of rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string, a bit face on the bit body, a plurality of cutting structures mounted in sockets in the bit body and projecting from the face of the bit, and a number of openings in the face of the bit communicating with a passage in the bit body for supplying drilling fluid to the face of the bit for cooling and/or cleaning the cutting structures.
Each cutting structure may comprise a cutting element bonded, for example by brazing, to a carrier which may be in the form of a stud of steel or tungsten carbide which is received and located in a socket in the bit body. The bit body may be formed, for example, from steel or from a solid infiltrated tungsten carbide matrix.
Usually, the cutting structures are secured within their respective sockets by brazing, press-fitting or shrink-fitting. However these methods have certain disadvantages.
For example, shrink-fitting necessitates the heating of the bit body to enlarge the sockets, followed by subsequent cooling of the body after the cutting structures have been inserted in the sockets. However, it is found that the necessary heating of the bit body can result in distortion of the sockets, for example as a result of stress relief, so that the cutting structures cannot be inserted easily into the sockets even after the bit body has been heated. The bit body may also be subjected to heating by other processes, for example the application of a hard-facing to a steel body, which may also result in distortion of the sockets.
In the case where the cutting structures are secured in the sockets by brazing, the brazing process subjects the bit body and cutting structures to an additional heat cycle. In most cases the cutting structure comprises a cutting element brazed to a stud or post and the bond effected by some brazing alloys may deteriorate as a result of the heating cycle required to braze the stud within the socket, thus weakening the bond between the cutting element and the stud.
In both brazing and shrink-fitting processes, the temperature of the bit body normally requires to be raised to temperatures above 450.degree. C., and at such temperatures the structure of the steel of the bit body begins to change and it begins to lose its tensile strength. This is obviously undesirable.
The present invention sets out to overcome these problems by using a method of securing the cutting structures within their sockets which does not require an elevated temperature, or which requires a temperature which is sufficiently low to avoid distortion of the sockets or deterioration of the bond between the cutting elements and the studs.